A diamond on the rough

Nobody likes a concrete road. England has 172 miles of concrete trunk roads, and promises have been made time and again to get rid of them all. But perhaps there's another solution.

You know a concrete motorway when you drive on one. The whining, droning noise from the tyres. The loud rhythmic thumps and jolts as you pass over the expansion joints. It's so unpleasant that the AA report some motorists thinking there must be something wrong with their car. Perhaps the only good thing is that heavenly feeling as you pass over the last jarring expansion joint, crossing onto black tarmac again, and peace and quiet returns.

It's been a while since we built concrete motorways. For many years, it was routine to invite two types of tender for construction of a new road - one for flexible (tarmac) surfaces and one for concrete - and see which was cheapest to build and maintain. But in the last twenty years concrete has been so unpopular that it's not even a contender any more, and since the late 1990s there have been periodic promises to get rid of all the concrete surfaces remaining on the trunk road network.

It's starting to look like there's another way that won't involve replacing the concrete at all. But before we look at that, we should probably ask why concrete roads were built in the first place.

Why concrete?

In the late 1930s, believing themselves to be on the brink of a huge (and lucrative) roadbuilding boom, the Cement and Concrete Association published a book called "Construction of Concrete Roads" that set out modern construction methods and gave their product the hard sell. They set out the reasons why concrete was then (and still is) a good material for road construction.

"The advantages of concrete - low initial cost, low maintenance charges, durability, safe surface for motorists, etc. - have proved to apply equally to heavily trafficked highways as well as to the carriage-ways on housing estates."

By the time motorways came along the plain facts hadn't changed. Where the tender price was lower than the cost of building and maintaining a tarmac road, the Ministry of Transport (and its successors) were happy to see a concrete road built. It often cost a bit more, but in the longer term a saving would be made on maintenance.

A motorway with a flexible surface, for example, might need patching or resurfacing of heavily worn areas after about a decade: lane 1, which is heavily used by HGVs, will be resurfaced relatively often. A full new surface might be required after twenty years, and 30-40 years down the line the whole surface might need to be stripped off and several sub-layers might have to be rebuilt.

A motorway with a concrete surface? Find yourself a hobby. It'll carry on for decades.

The fact that concrete is low-maintenance explains why it's popular in countries with vast road infrastructure but little government money for upkeep. The USA is the obvious example. Oddly, though, while concrete freeways are rife in the USA, they don't have the dreadful reputation they have here. We'll come back to that.

Falling out of love

Concrete roads have three key properties that mean they are unlike flexible pavement surfaces, and which cause all the things that people don't like.

Concrete is (usually) laid in sections.

If you pour a continuous line of concrete to make your road, it will crack, which is bad. Concrete cracks because it's rigid and doesn't cope well with the expansion and contraction it experiences through changing weather conditions. So, to prevent cracks, it's laid in small segments, which butt up against each other at expansion joints, giving the slabs room to expand without cracking. All of which is fine, until you drive over them at speed and the joints start rhythmically thumping under your wheels. The older and less well maintained the expansion joint, the noisier it is.

Concrete is ridged.

If you roll out a tarmac surface, you roll it flat. But a concrete surface that's perfectly flat is perfectly smooth, and nobody wants a glass-like road surface. It has no grip. So, after laying a concrete road, ridges and grooves are formed across its width. They're randomly spaced and at random depths, to prevent passing tyres creating a continuous whine of the same pitch, but that's little consolation: passing tyres will still create a continuous noise as they pass over the ridges, like a stick being dragged along iron railings.

Concrete is non-porous.

Unlike tarmac, which has lots of holes and a natural roughness to its surface, concrete sets solid and has very few natural fissures and crevices. On a tarmac road those air gaps absorb some of the noise from passing tyres, and tarmac doesn't make much noise anyway. But concrete does, thanks to those ridges - so the sound of tyres can be deafening. As concrete ages, it's polished by passing traffic and becomes smoother, generating even more noise.

Most of the trouble is, really, about noise. Towards the end of the era in which concrete was still used for new roads in the UK, manufacturers came up with something called "whisper concrete" to address it. The Highways Agency announced in 1996 that it would be mandatory on new concrete roads carrying more than 75,000 vehicles per day, and trials were successfully carried out where whisper concrete was laid on the M18 in Yorkshire and the A50 in Derbyshire.

Whisper concrete was a last gasp for concrete roads. People didn't like them and their minds were already made up. Time and again, the public expressed a preference for tarmac.

Papering over the cracks

Since the late 1990s, there have been repeated pledges to get rid of concrete on our road network - popular announcements, but not necessarily followed up with energetic action. The latest was in December 2017 from Highways England.

The proposal has been the same every time: lay a thin course of tarmac over the concrete. That gets rid of the surface noise and the expansion joint noise at the same time, which is fine, except that it's a time-limited solution.

Tarmac will, naturally, need renewal much sooner than concrete, so laying it over a concrete road introduces a new maintenance overhead for ever more. It also won't stick quite as well as it would on a road bed designed for tarmac, and after a few years will begin to break up around the expansion joints, requiring patching or resurfacing even sooner.

Nonetheless, this solution has been used many times, sometimes even on brand new roads. In 1999, a new length of M1 opened around the east side of Leeds, one of the very last concrete motorways to be built. Within weeks of opening local residents in the towns and villages it passed were complaining about the unbelievable levels of traffic noise from the new motorway - all the more incredible because, in its early days, it really wasn't all that busy.

The solution was to lay low-noise tarmac on sections of the brand-new concrete that were within a certain distance of residential areas. Today, if you drive the M1 between junctions 43 and 47, you'll pass back and forth between tarmac and concrete over and over again. And today, of course, you can start to see it deteriorating, with long thin patches covering places the tarmac has already broken up over the expansion joints.

Experience with over-laying concrete has clearly proven it's not a good way to get rid of concrete road noise. It's time to look at a different solution that doesn't involve replacing concrete at all.

Diamonds are forever

If you drive some of those concrete freeways in the USA, you might be surprised at how different they are to a concrete motorway. They don't make nearly as much noise. There is a reason for that.

Unlike in the UK, where a concrete surface is left alone and deteriorates slowly over decades, increasing in the surface noise it generates, concrete roads in many American states are treated periodically with a process called diamond grinding.

It means exactly what it says - a machine with diamond-tipped blades grinds the surface of the concrete, cutting new ridges - this time in line with the direction of travel, not across it. That has the effect of re-levelling the surface and making it less coarse. The grinder can also level out any bumps or differences in level between adjacent slabs, if any have formed. A concrete surface can be re-ground like this several times, perhaps decades apart, before it needs replacement.

The result is that your long-lasting concrete surface can remain, rather than being overlaid with something shorter-lived, maintained in a state that makes it smooth and quiet to drive.

Back in 2009, the Highways Agency decided to give it a go. In a trial overseen by the Transport Research Laboratory, several short sections of road were subjected to diamond grinding, and then tested long-term for the effect on noise, grip and durability.

The first to be done was a section of the A12 Chelmsford Bypass, near to Boreham, and a further experiment was made on the A14 near Ipswich. The results were good - as they should be, for a technique already routinely used for decades in the US - and the cost was said to be half that of overlaying the same length of road with tarmac. More impressively, skid resistance improved by 54%, while road noise reduced dramatically and noticeably.

Things have gone a bit quiet since then, but if you've driven in to London on the M1 you might have noticed something quietly happening near Watford. A particularly noisy concrete surface on the southbound side between junctions 5 and 6 became, in 2018, a bit less noisy. The only hint at what happened is a blue sign that reads "M1 J6-5 Bricket Wood to Watford Concrete Surfacing Treatment Trials Sept 2018-2022".

It's fairly clear - if you have the chance to glance down at the surface - that the horizontal ridges have been replaced with smaller longitudinal grooves, and the cats' eyes have been pulled out and replaced with stick-on road studs, meaning a diamond grinding machine has paid a visit to Hertfordshire. It's also clearly a long-term trial, the outcome of which might not be known for another three years.

What's interesting is that, since this trial started, Highways England has not just gone quiet about resurfacing concrete motorways - it's actually started kicking resurfacing projects into the long grass. One example is the M27 between junctions 5 and 7, now undergoing a Smart Motorway upgrade, where Highways England announced earlier this year that the road would not be resurfaced:

Our recent surveys have revealed that the concrete here is still fit for purpose… However, Highways England recognise there are some benefits to resurfacing the concrete section with a low noise surface and we commit to doing this after we have completed the smart motorway works. We are working to find the best way to deliver this.

"Working to find the best way to deliver this" suggests that rolling a thin layer of tarmac over the top might not be the only game in town.

Another potential candidate is the M25's southwestern section, which has the most appallingly noisy and bumpy surface. Resurfacing is off the cards, because the motorway there is maintained for Highways England by a third-party company whose contract doesn't cover the higher maintenance burden of a tarmac overlay. Diamond grinding might be an economical way to fix the surface, and would leave the hard-wearing concrete intact. We will see.

2022 is a long time to wait for results. Perhaps the benefits will be recognised and the trial wrapped up sooner than that. Whether or not that happens, it is starting to look like Highways England might not be getting rid of concrete roads at all; it might instead be doing something much more sensible, which is taking the concrete roads it has and making them pleasant to drive on.

They may, one day, turn around the reputation of concrete roads in the UK. They might even start building concrete roads again.

Well, maybe.